This invention relates to a method of controlling a wire bonding apparatus used in a step for assembling a semiconductor device.
For a wire bonding apparatus used for connecting bonding pads of a semiconductor chip and inner leads using bonding wires, apparatus of the type which joins or connects wires using ultrasonic wave are generally used. In such bonding apparatus using ultrasonic wave, the optimum conditions of pressure application time and/or pressure force of the ultrasonic wave vary depending upon the form or configuration, material or the like of the wire or the joining portion.
When joining is implemented using hard wires such as Cu wires, a large pressure force of 100 to 200 g is required. When joining is implemented to a broad joining portion using thick wires, a large pressure force of 100 to 200 g is also required. On the contrary, when joining is implemented to a small joining portion in order to carry out high density wiring, it is required to reduce a pressure force applied to the joining portion.
When an attention is made to the optimum condition for ultrasonic wave bonding in the wire bonding apparatus, it is required to take into consideration the area of the wire tip portion joined to the joining portion. This is because whether or not the condition for ultrasonic bonding is optimum is not determined by the entire energy applied to the joining portion by ultrasonic wave, but by the energy per unit area. Accordingly, if the joining area of the wire tip portion is too small, the energy per unit area becomes too large, with the result that bonding pads of semiconductor elements to be joined are damaged.
On the contrary, if the joining area of the wire tip portion is too large, the energy per unit area becomes too small, resulting in insufficient joining strength. In this case, if the ultrasonic energy is raised for the reason why the joining strength is insufficient, the tip portion of the wire is collapsed or destroyed to become too large, thus being in contact with a wire close thereto.
However, in the conventional wire bonding apparatus, the pressure force applied is fixed from the time when the bonding tool is positioned away from the joining surface by a distance greater than a fixed distance required until it reaches the joining surface to the time when the bonding tool is raised to the position away from the joining surface by more than fixed distance after joining or bonding has been completed. For this reason, if the tip portion of the wire varies in dimension, the pressure energy applied per unit area changes, resulting in the problem that the bonding cannot be necessarily conducted in the optimum condition at all times.
Where the pressure force required is large, it is possible to increase a speed for moving the bonding tool in upper and lower directions. However, where the pressure force required is small, there occurs the problem that the bonding tool vibrates when an attempt is made to move it in upper and lower directions at a high speed. For this reason, the bonding tool must be moved at a low speed when the pressure force is small, leading to the problem that it takes much time for bonding.
In addition, where joining is implemented by a large pressure force, there is the possibility that a large impact is given to the semiconductor chip to damage it, resulting in the problem that the tip portion of the wire is collapsed or destroyed.